Deserts can take many forms, including sand dunes, rocky canyons, sagebrush plains, and polar ice fields. But they are united by one thing: the lack of rain. Generally speaking, a desert is anywhere that receives less than 10 inches (25 centimeters) of rain per year. Lynn Fenstermaker (opens in new tab)Ecologist at the Desert Research Institute in Reno, Nevada.
Of course, the lack of rain means that the desert is famously dry. But why do some places on Earth get much less rain than others? In other words, why are deserts dry?
Global air circulation patterns are the biggest reason, Fenstermaker said. Solar energy hits the Earth most directly at the equator, heating the air and evaporating moisture from it. That warm, dry air rises and moves toward the poles. It dips again around 30 degrees latitude, Fenstermacher explained. This circulation pattern is called the Hadley Cell, and it drives the trade winds, which fueled the early exploration of the world by ocean explorers. It is also why many of the world’s largest deserts – viz Support and the Gobi in the Northern Hemisphere and the Kalahari in the Southern Hemisphere – these are at mid-latitudes.
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But the story is more complicated than that. Wind patterns interact with topography to influence where deserts are found. For example, air that passes over the ocean and hits a mountain range will drop its moisture on the slopes as rain or snow as the air rises. But when the air crosses the mountains and sinks down the other side, it dries up. In California, for example, the Mojave Desert sits in the rain shadow of the Sierra Nevada, Fenstermaker said.
Sometimes, inland areas are drier because they are so far from large bodies of water that they lose all their moisture by the time air arrives, he said. Andreas Prien (opens in new tab), an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colorado. This is the case in Central Asia’s Gobi Desert, which is also shielded by the Himalayas.
On the other hand, coastal does not always mean wet. Cold ocean currents colliding with coastal air can create fog. When that fog moves over land, the moisture stays in the air instead of falling as rain. This can lead to the formation of deserts around the sea, viz Atacama in Chile, one of the driest places on earth.
Not all deserts are hot either; Parts of the Arctic and Antarctic are considered deserts. Prine said that cold air cannot hold moisture as well as warm air. So cold temperatures at the poles lead to very little precipitation, even though enough water is stored in the ground as snow.
As global climate patterns change, so do deserts. For example, thousands of years ago, The Sahara was covered in grasslands and tropical forests (opens in new tab). And today, climate change is reshaping desert boundaries around the world.
“The Hadley Cell is expected to spread northward and southward due to climate change,” Prine said, extending the zone ripe for desert formation. Warmer temperatures can accelerate the shift by increasing water evaporation and drier air. Beyond just precipitation, it’s the balance of precipitation and evaporation that defines a desert, Prine added.
“Globally, with warming, what we expect is that we’re going to see more evaporation and just expand the existing desert regions,” Fenstermacher noted.
Human pressure on landscapes is also contributing. Cutting down trees to plant crops removes native vegetation, and some research suggests that Deforestation in the tropics is reducing rainfall (opens in new tab). If more water evaporates instead of being held by plants in the soil, a feedback loop dries out landscapes further. Semiarid areas bordering existing deserts are particularly vulnerable.
“It’s often a combination of factors that help deserts grow,” Prine said. “It’s not just human activity, or climate change, or natural climate variability, but it’s one on top of the other that brings ecosystems to a tipping point.”
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